The present invention relates to a process for obtaining light olefins from methanol and/or dimethyl ether (DME).
The catalytic conversion of methanol to hydrocarbons was first achieved by Mobil Oil Co. USA, over ZSM-5 zeolite catalyst in 1976, thereby a way of producing important raw materials, such as light olefins, from non-petroleum sources (coal and natural gas) was opened. This process is highly exothermic and the aimed product, light olefins, is further converted in the reaction zone. Consequently, the catalyst used was subjected to frequent regeneration and was easily damaged. Accordingly, various catalytic and engineering technologies have been explored with the purpose to control the reaction course, to enhance the selectivity for light olefins and to improve the stability of the catalyst. They include processes with different type of reactors, such as fixed or fluidized bed reactors, a process which yields a partial conversion at low temperature (&lt;350.degree. C.) with recycling, a process which yields a complete conversion at high temperature (&gt;450.degree. C.) without recycling, the reaction over different zeolite catalysts modified with various additives and also the reactions in the presence of some diluents in the feed.
In U.S. Pat. No. 4,035,430 a process for the production of gasoline range products from methanol over a ZSM-5 zeolite catalyst is disclosed. Methanol is catalytically converted to an ether rich product, and the latter is converted to aromatics and iso-paraffins. The exothermic temperature rise in the zeolite bed is kept low by mixing the ether-rich feed with a heat-dissipating material (hydrocarbon C.sub.1 -C.sub.5, cooled methanol etc.).
In U.S. Pat. No. 4,542,252, a multi-stage fixed bed adiabatic reactor system for methanol conversion to light olefins is disclosed. Also in this case the methanol feed is first converted into an ether-rich equilibrium mixture in a catalytic dehydration reactor. Thereafter it is further converted in a catalytic cracking reactor so that the released reaction heat can be distributed over the two conversion reactors. Furthermore the zeolite catalyst in the cracking reactor is packed in several stages separately, and the reaction heat is removed directly or indirectly by coolant or cool feed passing through the interstage space in order to keep the inlet temperature and temperature rise of each bed stage substantially equal.
In U.S. Pat. Nos. 3,911,041, 4,049,573, 4,100,219 JP 60-126233, JP 61-97231, JP 62-70324 and EP 6501, the zeolite catalysts used for the conversion of methanol to olefins are chemically modified with phosphorus, magnesium, silicon and alkali metal elements to control the acidity. These catalysts are mostly used at moderate temperature in reaction-separation recycle process. The methanol conversion in a single cycle is only 15-50% and the duration of individual reaction cycle is not long.